feat: add default_apps field to coder_agent resource

[sreya]

• Enables a template admin to configure which apps are displayed to the user in the dashboard.

fixes the provider side of coder/coder#8033

It takes the form

resource "coder_agent" "dev" {
	os = "linux"
	arch = "amd64"
	display_apps {
		vscode = false
		vscode_insiders = true
		web_terminal = false
		port_forwarding_helper = false
		ssh_helper = false
	} 
}

• If you do not add a display_apps block you opt-in to the defaults.
• To opt out of all apps you must explicitly set them all to false.

[mtojek]

Do we need an extra test case to check .tf with only a few apps selected:

display_apps {
   vscode = true
   web_terminal = true

[sreya]

Since they all have default values I tested that specifying their non-defaults (in OK) and then omitting them to produce the defaults (in Omitted) produces the desired result. I added another one anyway but I don't think it adds much value.

[mtojek]

👍

just take a look at the comment in examples_test.go